Using modern imaging methods, two- or three-dimensional image data is frequently produced which can be used for visualizing an examination object and, in addition, for other applications also.
The imaging methods are often based on the detection of X-radiation, wherein so-called projection measurement data is generated. Projection measurement data can be acquired, for example, using a computed tomography system (CT system). In CT systems, a gantry-mounted combination of X-ray source and oppositely disposed X-ray detector usually revolves around a scanning chamber in which the examination object (which, without limitation of generality, will be referred to as the patient in the following) is positioned. The center of rotation (also termed the “isocenter”) coincides with a so-called system axis z. In the course of one or more revolutions, the patient is irradiated with X-radiation from the X-ray source, wherein the X-ray detector opposite is used to acquire projection measurement data or more specifically X-ray projection data.
The X-ray detectors employed in CT imaging usually have a plurality of detection units which are mostly arranged in the form of a regular pixel array. The detection units each generate, for the X-radiation incident on the detection units, a detection signal which is analyzed at particular points in time in respect of intensity and spectral distribution in order to draw conclusions about the examination object and to produce projection measurement data.
For imaging body structures of patients, so-called contrast agents are frequently used. However, before contrast-agent-based medical imaging can commence, it must be ensured that, having been injected into the patient's body, the contrast agent is also in the area of the patient's body that is to be examined.
Often the point in time at which imaging is to commence is simply estimated on the basis of empirical values. However, such an approach is not particularly precise. If the scan start time is set too late, this increases the total length of time for which the contrast agent is in the patient. However, the aim is basically to achieve a maximally short residence time of the contrast agent in the body, as the contrast agent can be harmful to the human body. If imaging is started too early, this may result in poorer image quality. In the worst case scenario, imaging and also the administration of contrast agent will even have to be repeated, which places an additional burden on the patient.
One option for making the spread of the contrast agent in the body visible prior to actual imaging involves carrying out a so-called bolus tracking scan (BT scan for short) which is performed prior to actual imaging. Such a BT scan can be a low-resolution, time-dependent scan, e.g. a CT scan with which a time-density curve of a sub-region of a region of interest is acquired.
Such a sub-region for a BT scan usually encompasses a slice which is formed and also viewed orthogonally to the z-direction, i.e. the direction of the system axis of the imaging system. Specifically in the case of the BT scan, attenuation values are acquired as a function of time and space in a sub-region of the region of interest in which usually an artery is located. If the injected contrast agent now flows through the observed artery, the attenuation values are significantly increased. If a predetermined limit value of the attenuation values is exceeded, e.g. 150 Hounsfield units (HU), this can be interpreted as indicating that the contrast agent is present in sufficient concentration in the region of interest, and the actual imaging examination can commence. For such a bolus scan, the position of a slice to be imaged in the bolus scan must be determined prior to the actual imaging.
In general, for CT image acquisition, an extent of a region to be imaged in the z-direction, also referred to as a scan range in the following, and a width of the region to be imaged which defines the so-called field of view (FoV), and possibly also the position of a slice to be imaged in a bolus scan are determined in advance. For this purpose a so-called topogram is generally obtained. This is a two-dimensional X-ray projection of the patient. On the basis of the topogram, the scan start position, the end position of the scanning process, hereinafter amalgamated as the scan range, and the field of view as well as possibly the positioning of a bolus scan slice are determined by technical personnel or using an automated algorithm.
However, the topogram-based planning of a CT imaging sequence means additional workload. In addition, the patient has to wait for a longer time in the CT system to enable the topogram to be obtained. Moreover, the acquisition of a topogram places an additional radiation load on the patient.